Test code using JUnit in JVM – tutorial

1. Open a Kotlin project in IntelliJ IDEA. If you don't have a project, create one.

2. Open the build.gradle(.kts) file and check that the testImplementation dependency is present. This dependency allows you to work with kotlin.test and JUnit:

   Kotlin

   Groovy

   dependencies {
       // Other dependencies.
       testImplementation(kotlin("test"))
   }

   dependencies {
       // Other dependencies.
       testImplementation 'org.jetbrains.kotlin:kotlin-test'
   }

3. Add the test task to the build.gradle(.kts) file:

   Kotlin

   Groovy

   tasks.test {
       useJUnitPlatform()
   }

   test {
       useJUnitPlatform()
   }

   note

   If you use the useJUnitPlatform()function in your build script, the kotlin-test library automatically includes JUnit 5 as a dependency. This setup enables access to all JUnit 5 APIs, along with the kotlin-test API, in JVM-only projects and JVM tests of Kotlin Multiplatform (KMP) projects.

Here's a complete code for the build.gradle.kts:

plugins {
 {...}

1. Open the Main.kt file in src/main/kotlin.

   The src directory contains Kotlin source files and resources. The Main.kt file contains sample code that prints Hello, World!.

2. Create the Sample class with the sum() function that adds two integers together:

   class Sample() {
   
       fun sum(a: Int, b: Int): Int {
           return a + b
       }
   }

1. In IntelliJ IDEA, select Code | Generate | Test... for the Sample class:

   

2. Specify the name of the test class. For example, SampleTest:

   

   IntelliJ IDEA creates the SampleTest.kt file in the test directory. This directory contains Kotlin test source files and resources.

   note

   You can also manually create a *.kt file for tests in src/test/kotlin.

3. Add the test code for the sum() function in SampleTest.kt:

   • Define the test testSum() function using the @Test annotation.

   • Check that the sum() function returns the expected value by using the assertEquals() function.

   import org.example.Sample
   import org.junit.jupiter.api.Assertions.*
   import kotlin.test.Test
   
   class SampleTest {
   
       private val testSample: Sample = Sample()
   
       @Test
       fun testSum() {
           val expected = 42
           assertEquals(expected, testSample.sum(40, 2))
       }
   }

1. Run the test using the gutter icon:

   

   note

   You can also run all project tests via the command-line interface using the ./gradlew check command.

2. Check the result in the Run tool window:

   

   The test function was executed successfully.

3. Make sure that the test works correctly by changing the expected variable value to 43:

   @Test
   fun testSum() {
       val expected = 43
       assertEquals(expected, classForTesting.sum(40, 2))
   }

4. Run the test again and check the result:

   

   The test execution failed.

Once you've finished your first test, you can:

• Write more tests using other kotlin.test functions. For example, use the assertNotEquals() function.

• Improve your test output with the Kotlin Power-assert compiler plugin. The plugin enriches the test output with contextual information.

• Create your first server-side application with Kotlin and Spring Boot.